Android 把从网络获取的图片缓存到内存中

1：activity_main.xml

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent">

    <Button
        android:id="@+id/btn_1"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:text="Button test1"/>

    <Button
        android:layout_below="@id/btn_1"
        android:id="@+id/btn_2"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:text="Button Intent Other Activity"/>

    <ImageView
        android:layout_below="@id/btn_2"
        android:id="@+id/img_view"
        android:layout_width="match_parent"
        android:layout_height="match_parent"/>

</RelativeLayout>

2:HttpHelper.java

public class HttpHelper {
    //图片资源缓存
    private static Map<String,Bitmap>bitmapCache=new HashMap<String,Bitmap>();

    public static Bitmap getHttpBitmap(String url){
　　　　　//首先先从缓存中取数据
        Bitmap bitmap=bitmapCache.get(url);
        if(bitmap!=null){
　　　　　　　　//如果取到就直接返回
            return bitmap;
        }

        try{
            URL myUrl=new URL(url);
            HttpURLConnection conn=(HttpURLConnection)myUrl.openConnection();
            conn.setDoInput(true);
            conn.connect();
            InputStream is=conn.getInputStream();
            bitmap=BitmapFactory.decodeStream(is);
            is.close();
        }catch(Exception e){
            e.printStackTrace();
        }

        if(bitmap!=null){
　　　　　　　　//将获取到的图片缓存起来
            bitmapCache.put(url, bitmap);
        }

        return bitmap;
    }
}

3：MainActivity.java

public class MainActivity extends Activity {
    private Button btnTest1=null;
    private Button btnTest2=null;
    private ImageView imgView=null;
    private Bitmap bitmap=null;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        initUI();

        btnTest1.setOnClickListener(new OnClickListener(){
            public void onClick(View view){
                new Thread(new GetImgThread()).start();
            }
        });

        btnTest2.setOnClickListener(new OnClickListener(){
            public void onClick(View view){
                Intent intent=new Intent(MainActivity.this,OtherActivity.class);
                startActivity(intent);
            }
        });
    }

    private void initUI(){
        btnTest1=(Button)findViewById(R.id.btn_1);
        btnTest2=(Button)findViewById(R.id.btn_2);
        imgView=(ImageView)findViewById(R.id.img_view);
    }

    Handler myHandler=new Handler(){
        public void handleMessage(Message msg){
            imgView.setImageBitmap(bitmap);
        }
    };

    class GetImgThread implements Runnable{
        public void run(){
            String url="http://upload.wikimedia.org/wikipedia/commons/thumb/e/ea/Hukou_Waterfall.jpg/800px-Hukou_Waterfall.jpg";
            bitmap=HttpHelper.getHttpBitmap(url);
            myHandler.obtainMessage().sendToTarget();
        }
    }
}

4:activity_other.xml

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent" >

     <Button
         android:id="@+id/btn_get"
         android:layout_width="match_parent"
         android:layout_height="wrap_content"
         android:text="Button Get Img"/>

    <ImageView
        android:id="@+id/img_view_2"
        android:layout_below="@id/btn_get"
        android:layout_width="match_parent"
        android:layout_height="match_parent"/>
</RelativeLayout>

5:OtherActivity.java

public class OtherActivity extends Activity {
    private  Bitmap bitmap=null;
    private Button btnGetImg=null;
    private ImageView imgView=null;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_other);

        btnGetImg=(Button)findViewById(R.id.btn_get);
        imgView=(ImageView)findViewById(R.id.img_view_2);

        btnGetImg.setOnClickListener(new OnClickListener(){
            public void onClick(View view){
                new Thread(new GetImgThread()).start();
            }
        });
    }

    Handler myHandler=new Handler(){
        public void handleMessage(Message msg){
            imgView.setImageBitmap(bitmap);
        }
    };

    class GetImgThread implements Runnable{
        public void run(){
            String url="http://upload.wikimedia.org/wikipedia/commons/thumb/e/ea/Hukou_Waterfall.jpg/800px-Hukou_Waterfall.jpg";
            bitmap=HttpHelper.getHttpBitmap(url);
            myHandler.obtainMessage().sendToTarget();
        }
    }

}

6:运行结果如下：